Aspects of the present disclosure relate generally to electric power conversion circuits for driving gas discharge lamps, and in particular to relamping circuits for use in ballast circuits for gas discharge lamps.
Heated filament gas discharge lamps, such as the fluorescent lamp common in homes and commercial buildings, are a type of electric light generating device that create light by passing an electric current through a mixture of gases contained within a sealed tube or bulb. To initiate light production, or ignite the lamp, filaments at ends of the tube are heated and a relatively high voltage, known as an ignition voltage, is applied across the lamp to ionize the gases and initiate an arc within the lamp tube. Once an arc has been established and the filaments have warmed enough to sustain thermionic emissions, the lamp enters a steady state where light production can be maintained with a lower voltage. During steady state operation, gas discharge lamps exhibit phenomena known as negative resistance, where increasing current results in lower electric resistance. This negative resistance can create an unstable current condition, that if left unchecked, will destroy the lamp. To overcome this problem, gas discharge lamps are typically driven with current limiting driver circuits that prevent high currents from damaging the lamps. These current limiting driver circuits are known as ballast circuits or ballasts.
A common type of ballast circuit used to drive fluorescent lamps is a resonant inverter circuit. Resonant inverters have properties that are particularly well suited to driving gas discharge lamps. For example, resonant inverters can provide the relatively high ignition voltages, can control current delivered to the lamps, and can provide improved lamp life. These resonant inverters typically receive a DC voltage and use a set of switching devices to apply an AC voltage to a resonant LC circuit to produce a high frequency lamp power. The voltage of the lamp power can be easily regulated by adjusting the frequency of the AC voltage, while current is easily controlled by proper selection of a capacitor size. As the frequency of the AC voltage is moved closer to or farther away from the resonant frequency of the resonance of the LC circuit, the voltage of the lamp power is increased or reduced respectively.
It is often desirable to replace lamps in light fixtures without turning the fixtures off. To overcome this problem many lamp ballasts include relamping circuits that sense failed or removed lamps and shut down the ballast and restart the ballast when a new lamp is installed. However, many fixtures used a single ballast to drive multiple lamps and this approach makes it difficult to determine which lamp failed and can also reduce light levels around the fixture making it difficult to install new lamps. Typical relamping circuits also use a high number of components thereby increasing costs and lowering reliability.
Accordingly, it would be desirable to provide a relamping circuit topology that solves at least some of the problems identified above.